Published: Journal of Analytical Toxicology, ISSN 0146-4760, Volume 25, Number 7, October, pp. 531-537

Simultaneous Determination of D⁹-Tetrahydrocannabinol and 11-nor-9-Carboxy-D⁹-Tetrahydrocannabinol in Human Plasma by Solid-Phase Extraction and Gas Chromatography–Negative Ion Chemical Ionization-Mass Spectrometry
Wei Huang, David E. Moody, David M. Andrenyak, Elizabeth K. Smith, and Rodger L. Foltz
Center for Human Toxicology, University of Utah, Salt Lake City, Utah 84112
Marilyn A. Huestis
Chemistry and Drug Metabolism, Intramural Research Program, National Institute on Drug Abuse, Baltimore, Maryland 21224
John F. Newton
Clinical Metabolism and Pharmacokinetics, Sanofi-Synthelabo Pharmaceuticals, Inc., Malvern, Pennsylvania 19355

D⁹-Tetrahydrocannabinol (THC) and 11-nor-9-carboxy-D⁹-tetrahydrocannabinol (THCA) in human plasma can be simultaneously detected using solid-phase extraction with gas chromatography and negative ion chemical ionization mass spectrometry. THC-d₃ and THCA-d₃ are added as internal standards; protein is precipitated with acetonitrile and the resulting supernatants diluted with 0.1M sodium acetate (pH 7.0) prior to application to the solid-phase extraction columns. THC and THCA were eluted separately and then pooled, dried under air, and derivatized with trifluoroacetic anhydride and hexafluoroisopropanol. The derivatized THC-d0 gives abundant molecular anions (m/z 410), and the derivatized THCA-d₀ gives abundant fragment ions (m/z 422) formed by loss of (CF3)2CHOH from its molecular anion. The recoveries of THC and THCA were 74% and 17%, respectively. The lower and upper limits of quantitation were 0.5 and 100 ng/mL for THC and 2.5 ng/mL and 100 ng/mL for THCA. The within-run accuracy and precision for THC (measured at 0.5, 1, 10 and 75 ng/mL) ranged from 98 to 106% (% target) and 4.1 to 9.5 (%CV), respectively. For THCA, the within-run accuracy and precision (measured at 2.5, 5, 10, and 75 ng/mL) ranged from 89 to 101% and 4.3 to 7.5%, respectively. The between-run accuracy and precision for THC ranged from 92 to 110% and 0.4 to 12.4%, respectively. The between-run accuracy and precision for THCA ranged from 97 to 103% and 6.5 to 12.3%, respectively. In processed samples stored in reconstituted form at –20°C, THC and THCA were stable for at least three days. THC and THCA stored in plasma were stable following three freeze/thaw cycles. THC and THCA in whole blood at room temperature for 6 h, or in plasma stored at room temperature for 24 h, did not show significant change. Storage in polypropylene containers for 7 days at –20°C and the presence of 1% sodium fluoride or the cannabinoid receptor antagonist, SR141716, at 1 µg/mL did not interfere with the quantitation of THC and THCA. In three individuals who smoked marijuana under controlled dosing conditions, peak THC concentrations of 151, 266, and 99 ng/mL were seen in the first plasma samples drawn immediately after the end of smoking, and corresponding peak THCA concentrations of 41, 52, and 17 ng/mL occurred at 0.33 to 1 h after cessation of smoking.

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